The autoimmune lymphoproliferative syndrome (ALPS) is a human disorder due to defective lymphocyte apoptosis resulting in abnormalities in lymphocyte homeostasis. This produces a combination of lymphadenopathy, autoimmunity and increased risk of lymphoma. The diagnostic criteria used to diagnose ALPS include a triad of findings: lymphadenopathy, increased circulating alpha-beta double negative T cells and defective in vitro Fas mediated lymphocyte apoptosis. The majority of patients with ALPS have been found to have a heterozygous mutation in the gene encoding Fas (CD95) while a small number of patients have been found to have mutations in Fas ligand or caspase 10. However, a sizeable number of ALPS patients do not have mutations in the genes noted about and are categorized as having ALPS type 3 with an uncharacterized defect. We are systematically evaluating ALPS type 3 patients by sequentially studying intracellular proteins involved with the apoptotic process. These are targeted at assessing the assemblage of the Death Inducing Signalling Complex (DISC) and downstream events and proteins that control the apoptotic cascade that ultimately results in cell death. The initial identification of two ALPS type 3 patients with increased levels of an inhibitory protein, FLIP (Flice Inhibitory Protein) that is involved in the regulation of DISC mediated caspase activation continue focused on the control mechanism(s) involved in FLIP regulation and attempting to replicate the defect seen in these patients by using siRNA knock down techniques in normal lymphocytes. These studies are being reactivated after focusing our primary energies in a patient with a defect in an intrinsic apoptotic pathway that is tthe subject of an alternative project. The focus of the studies in the future will be to identify the frequency with which alternations in FLIP are seen in ALPS type 3 as well as to define if any other downstream defects in the extrinsic apoptotic pathway are present in ALPS type 3.